[hotfix][docs] Add that LIMIT requires ORDER BY.

[fhueske]

What is the purpose of the change

Fixes the documentation of SQL LIMIT which requires the ORDER BY clause.

Brief change log

• add a note explaining that LIMIT requires ORDER BY
• fix example

Verifying this change

• build docs and check manually.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? n/a

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Review Progress

• ✅ 1. The [description] looks good.
  • Approved by @sunjincheng121 [committer]
• ✅ 2. There is [consensus] that the contribution should go into to Flink.
  • Approved by @sunjincheng121 [committer]
• ❓ 3. Needs [attention] from.
• ✅ 4. The change fits into the overall [architecture].
  • Approved by @sunjincheng121 [committer]
• ✅ 5. Overall code [quality] is good.
  • Approved by @sunjincheng121 [committer]

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[sunjincheng121]

Thanks for the fix @fhueske!
LGTM. Merging.
@flinkbot approve all

[sunjincheng121]

Fixed in master: be9f030
Fixed in release-1.8: 77dc5de